Page 178 - Chemical process engineering design and economics

160 Chapter 4

For safety and for ease of manufacture, organizations are established to de-
velop standards and to facilitate the exchange of design information. The me-
chanical design of heat exchangers is governed by the Tubular Exchanger Manu-
facturers Association (TEMA) [19], the American Petroleum Institute (API) [21],
and the American Society of Mechanical Engineers (ASME) [20]. These organi-
zations publish standards and update them regularly.

Fluid Location

Locate the fluid on the tube side if the fluid is:

1. more corrosive
2. less viscous
3. more fouling
4. at a higher pressure
5. hotter
6. at a higher flow rate

and also if the fluid requires a low pressure drop. Generally, the more "obnoxious"
fluid is placed on the tube side because:

1. the tube side is relatively easy to clean
2. tubes are easier to replace or plugged if damaged
3. high heat-transfer coefficients can be obtained at a low pressure drop
4. a high-pressure fluid is more economically contained in tubes because of their
smaller diameter compared to the shell

Cooling water, for example, will be placed on the tube side because of its tendency
to form a scale. Water usually contains dissolved salts, like calcium carbonate,
which may deposit on the tube wall. A condensing fluid will be placed in the shell
side to prevent the liquid film from growing too large, reducing the heat-transfer
coefficient, or in the tube side if subcooling of the liquid is desirable. In the shell
side, turbulence occurs at a lower Reynolds number than in the tube side because
of the baffles. Thus, the shell side is the best location for very viscous fluids.
Heat-Exchanger Sizing

The well-known formula for sizing heat exchangers is

(4.1)
Q = U 0A 0(At) LM
where the subscript, o, signifies that the overall heat-transfer coefficient is based
on the outside tube area. Sizing a heat exchanger entails calculating the area re-

Copyright © 2003 by Taylor & Francis Group LLC